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Galactic hot spots may signal supernovas.

Tracking supernova remnants -- the luminous matter ejected when massive stars explosively collapse -- can help chart the evolution of galaxies. Astro-physicists now report a new technique for detecting these brilliant stellar objects. Using new, high-resolution infrared detectors that can home in on galactic hot spots, they scan for regions ablaze with infrared light.

Supernovas typically spew iron and other heavy elements into the surrounding space, irrevocably altering a galaxy's chemical composition. The associated shock wave can compress interstellar gas, an effect that might accelerate star formation. The shock also heats surrounding dust particles, causing them to glow brightly in the infrared.

Until recently, however, researchers lacked detectors sensitive enough to pin-point the location of these telltale infrared emissions, notes Duncan A. Forbes of the University of Cambridge in England. Ground-based searches for supernovas in nearby, dust-shrouded galaxies, for instance, had to rely almost exclusively on radio surveys.

But with the development of more sensitive infrared detectors in the 1980s, that scenario began changing. The new detectors contain hundreds of individual light sensors, each of which possesses a resolution far exceeding previous infrared sensors.

Colin A. Norman of the Space Telescope Science Institute in Baltimore and Dave Van Buren, now at the California Institute of Technology in Pasadena, suggested in 1989 that astronomers could use the new devices to examine supernova activity in starburst galaxies. These dusty galaxies produce copious numbers of new stars--including many massive objects likely to end their life as supernovas. And although dust permits only a small amount of visible light to reach Earth, near-infrared light passes through dust unimpeded.

Last year, Forbes and his co-workers imaged the nearby starburst galaxy NGC 253 using a large-format infrared detector at the Cerro Tololo Inter-American Observatory in La Serena, Chile. They were prompted in part by a 1988 report that the galaxy's nucleus contained several compact radio sources. The radio emissions hinted that NGC 253 might have a high rate of supernova explosions.

The team's investigation uncovered four previously unknown infrared hot spots near the center of the galaxy, Forbes says. Moreover, these active, infrared regions match the locations of the radio emissions, he and his group report in the Oct. 20 ASTROPHYSICAL JOURNAL LETTERS. These hot spots also coincide with radio sources detected in a follow-up, higher-resolution radio survey of NGC 253, reported by James S. Ulvestad of the Jet Propulsion Laboratory in Pasadena, Calif., and Robert R. J. Antonucci of the University of California, Santa Barbara, in the September ASTRONOMICAL JOURNAL, Forbes notes.

Forbes and his coauthors caution that the hot spots could result from activity not directly related to supernovas. For example, young supergiant stars and ionized hydrogen gas in the galaxy's interstellar medium also produce intense infrared emissions. However, unlike the spectra associated with supernova activity, these infrared signals would contain little radiation from ionized iron.

Since his initial study of NGC 253, Forbes told SCIENCE NEWS, he and a team that includes Reinhard Genzel of the Max Planck Institute for Physics and Astro-Physics in Garching, Germany, have detected infrared iron emissions from regions of the starburst galaxy that appear to coincide with the hot spots. The new, unpublished finding all but clinches confirmation that the infrared emissions result from supernova activity, he says.

Van Buren disagrees, arguing that the brilliance of these hot spots suggests star clusters produced the radiation, not a supernova. But he concurs with Forbes that the infrared detectors "are more sensitive and can do a more thorough job" of searching for radiation characteristic of recent supernova explosions than radio telescopes. The new infrared detectors also probe more quickly and can image sources in more distant galaxies than radio telescopes, Van Buren says. Like Forbes, he plans to continue searching for infrared hot spots that may signify supernova activity.
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Author:Cowen, Ron
Publication:Science News
Date:Oct 26, 1991
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